1. Field of the Invention
The invention relates generally to audio amplification systems, and more particularly to systems and methods for minimizing errors due to component variation in switching amplifiers utilizing power supply feed forward techniques.
2. Related Art
Practical audio power amplifiers using Pulse Width Modulation (PWM) have been known since the mid 1960s. In amplifiers from that era, a pulse train was generated by comparing a voltage representing the incoming audio signal with a reference waveform, typically a triangular wave or sawtooth wave, with a frequency in the range 50 kHz-200 kHz. The comparison yielded a 2-level rectangular wave having the same frequency as the reference waveform, and with a mark:space ratio varying in sympathy with the audio. The rectangular wave was amplified to the desired power level and then passively lowpass filtered to remove most of the high-frequency components of the rectangular wave, leaving its average level, which follows the audio, to drive a load such as a loudspeaker.
Amplifiers as described above have sometimes been called ‘digital’ in the popular press, but we shall describe them as ‘analog’, because the timings of the edges of the rectangular waves can vary continuously in sympathy with the audio. We shall reserve the word ‘digital’ for an amplifier in which the edge timings are quantized, so that the edge timings can be represented digitally and the edges can be generated by counting pulses produced by a high-precision, high-frequency clock, such as a crystal oscillator. This principle was proposed by Sandier [6], who also realized that the apparent need for a clock frequency in the gigahertz region could be avoided by the use of oversampling and noise shaping. Several commercial products are now available that use this principle (see for example [3].)
As conventional open-loop digital amplifiers have very little power supply rejection, they require high precision DC power sources to provide high fidelity audio. The construction of these power sources requires large and expensive capacitors and inductors to provide the high precision DC power sources. Smedley, “Digital, pulse width modulation audio power amplifier with noise and ripple shaping,” U.S. Pat. No. 5,559,467 (1995) illustrates how to correct for the power supply line ripple. Others, like Andersen et al, “Delta-sigma amplifiers with output stage supply voltage variation and compensations and methods and digital amplifier systems using the same,” U.S. Pat. No. 6,953,620 (2004) illustrate how to correct for power supply common mode imperfections.
Both of these examples, however are limited in their performance based on the quality of the analog to digital conversion process, and more specifically to the analog filter used on the input to the analog to digital converter (ADC).